重载 = 运算符时如何使用 THIS 指针

How to Use THIS pointer when Overloading = Operator

本文关键字：何使用 THIS 指针 运算符 重载      更新时间：2023-10-16

所以我让 += 运算符重载，试图利用我已经定义的 + 运算符。即多项式 + 多项式返回一个新的多项式。所以我的 += 函数基本上尝试调用这个 + 函数，LHS 为 *this，RHS 为 *this + B（其中 B 是通过常量引用传递给函数的多项式对象）。我收到错误。我哪里出错了？ 

#include <iostream>
#include <ctime>
#include <cstdlib>
#include <cmath>
using namespace std;
void line(int lines);

class Polynomial
{
    private:
        int degree;
        double* coeffs;
    public:
        //constructors
        Polynomial() {degree=0;coeffs=new double[1];}
        Polynomial(int deg) {degree=deg;coeffs=new double[deg+1];}
        Polynomial(const Polynomial& A);
        //mutators
        void GetCoeffs(istream& in);
        void EditCoeff(int deg);
        void ResetCoeffs();
        int Coeff(int deg);
        void Randomize(int max);
        //accessors
        void Show(ostream& out);
        int Degree() {return degree;}
        //operators
        Polynomial operator+(const Polynomial& B); //Polynomial + Polynomial
        friend Polynomial operator +(double c, Polynomial& A); //c + Polynomial
        Polynomial operator +(double c); //Polynomial + c
        void operator +=(const Polynomial& B); //Polynomial+=Polynomial
        void operator =(Polynomial& A);
        Polynomial operator*(int k);
        Polynomial operator*(Polynomial& A);
};

int main()
{
    Polynomial A(5);
    A.Randomize(4);
    A.Show(cout);
    line(2);
    Polynomial B=A+8;
    B.Show(cout);


    return 0;
}
Polynomial Polynomial::operator*(int k)
{
    Polynomial C(degree);
    C=*this;
    for (int i=degree; i>=0; i--)
        C.coeffs[i]*=k;
    return C;
}
Polynomial operator +(double c, Polynomial& A)
{
    Polynomial C=A;
    C.coeffs[0]+=c;
    return C;
}
Polynomial Polynomial::operator +(double c)
{
    Polynomial C=*this;
    C.coeffs[0]+=c;
    return C;
}
void Polynomial::Randomize(int max)
{
    for (int i=degree; i>=0; i--)
    {
        coeffs[i]=rand()%(max+1) + 1;
        if ((rand()%(101) + 1)%2 == 0)
            coeffs[i]*=-1;
    }
}
void Polynomial::operator =(Polynomial& A)
{
    if (degree==A.degree)
    {
        for (int i=degree; i>=0; i--)
        {
            coeffs[i]=A.coeffs[i];
        }
    }
}
Polynomial Polynomial::operator+(const Polynomial& B)
{
    if (degree>=B.degree)
    {
        Polynomial C(degree);
        C=*this;
        for (int i=B.degree; i>=0; i--)
        {
            C.coeffs[i]=coeffs[i]+B.coeffs[i];
        }
        return C;
    }
    else
    {
        Polynomial C=B;
        for (int i=degree; i>=0; i--)
        {
            C.coeffs[i]=coeffs[i]+B.coeffs[i];
        }
        return C;
    }
}
void Polynomial::operator+=(const Polynomial& B)
{
    *this = (*this + B);
}
int Polynomial::Coeff(int deg)
{
    return coeffs[deg];
}
void line(int lines)
{
    for (int i=0; i<lines; i++)
        cout << endl;
}
void Polynomial::GetCoeffs(istream& in)
{
    for (int i=degree; i>=0; i--)
    {
        in >> coeffs[i];
    }
    in.ignore();
}
void Polynomial::Show(ostream& out)
{
    if (coeffs[degree]>0)
                cout << "   ";
    for (int i=degree; i>=0; i--)
    {

        if (coeffs[i]>=0)
        {
            if (i!=degree)
                out << " + ";
            out << coeffs[i];

        }
        else
        {
            if (coeffs[i]<0)
                out << " - ";
                out << 0-coeffs[i];
        }
        if (i>1)
            out << "x^" << i;
        else if (i==1)
            out << "x";
    }
}
Polynomial::Polynomial(const Polynomial& A)
{
    coeffs=new double[A.degree+1];
    degree=A.degree;
    for (int i=A.degree; i>=0; i--)
    {
        coeffs[i]=A.coeffs[i];
    }
}
Polynomial Polynomial::operator*(Polynomial& A)
{
    int deg=A.degree+degree;
    Polynomial P(deg);
    for (int i=deg; i>=0; i--)
        P.coeffs[i]=0;

    for (int i=deg; i>=0; i--)
    {
        for (int j=A.degree; j>=0; j--)
        {
            P.coeffs[i+j]+=coeffs[i]*A.coeffs[j];
        }
    }
    return P;

}

您的问题是*this + B是临时的，临时对象不能绑定到非const引用。

当然，没有理由不能const作业的 RHS。 尝试：

void operator=(/* HERE */ const Polynomial& A);

您的大多数其他操作员也没有在他们应该使用的地方使用const。 例如：

Polynomial operator+(const Polynomial& B) /* HERE */ const; //Polynomial + Polynomial
friend Polynomial operator +(double c, /* HERE */ const Polynomial& A); //c + Polynomial

只有赋值运算符应该是非const成员，并且他们应该接受const正确的操作数。 创建新对象的普通二元运算符应同时const左操作数和右操作数。

通常以另一种方式完成。人们制作二进制+运算符（通常是非成员），利用已经定义的+=运算符（成员）。

我哪里出错了？

  • 应使用 std::vector 而不是原始指针。
  • 你违反了三法则。
  • 您没有遵循创建会员operator+=和免费operator+的约定。

试试这个：

// UNTESTED
class Polynomial
{
    private:
        std::vector<double> coeffs;
    public:
        //constructors
        Polynomial() : coeffs(1) {}
        Polynomial(int deg) : coeffs(deg+1) {}
        // Don't need copy constructor
        // Don't need destructor
        // Don't need assignment operator
        ...
        int Degree() {return coeffs.size()-1;}
        //operators
        Polynomial& operator+=(const Polynomial& B) {
            if(Degree() < B.Degree())
              coeffs.resize(B.Degree()+1);
            for(int i = 0; i <= B.Degree(); ++i)
              coeffs[i] += B.coeffs[i];
            return *this;
        }    
};
Polynomial operator+(Polynomial A, const Polynomial& B) {
  return A += B;
}
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